This invention relates to processes for preparing 2,2,4-trimethyl-1,3-pentanediol and derivatives thereof including its mono- and diisobutyrate esters. More particularly, the invention relates to alkali metal hydroxide catalyzed processes for such preparation which are unexpectedly and advantageously modified by the presence of alkali metal isobutyrate salts.
By way of further background, 2,2,4-trimethyl-1,3-pentanediol and its derivatives, including for example its mono- and diisobutyrate esters, have proven to be highly useful materials. For instance, they have been used as or as intermediates to plasticizers, pesticides, polyester resins, lubricants, printing inks, etc. Quite naturally, therefore, much study has focussed upon achieving and improving upon processes for the production of these chemicals.
For example, the base-catalyzed condensation of isobutyraldehyde to 2,2,4-trimethyl-1,3-pentanediol and corresponding esters, and particularly to its monoisobutyrate ester (3-hydroxy-2,2,4-trimethylpentyl isobutyrate, sometimes referred to herein as "the monoester"), has been the subject of continuing study throughout the years. Such studies have been reported extensively in the patent and other literature. For example, U.S. Pat. No. 3,091,632 issued to Hagemeyer et al. in 1963 reports the use of alkali metal alkoxide catalyst, under dry, acid free conditions, to catalyze the condensation of aldehydes such as isobutyraldehyde to glycol monoesters. The condensations are said to occur at a temperature of 65.degree. C. to 105.degree. C., a catalyst concentration 0.05 to 2 weight percent based on the aldehyde feed, and a residence time within the reaction zone in the range of 0.25 to 3 hours.
U.S. Pat. No. 3,291,821 to Perry et al. discloses aldehyde condensations catalyzed by alkali metal hydroxides or alkaline earth metal hydroxides. An aqueous solution of the base is intimately contacted with the aldehyde for a period of about 15 minutes to 2 hours at a temperature of about 50.degree. C. to 125.degree. C. The alkali metal hydroxide, sodium hydroxide, is preferred, with a 10% by weight aqueous solution typically being used at an organic to aqueous phase ratio of 85:15 to 75:25. The mixing of the aqueous and organic phases is accomplished by directing them against a baffle plate disposed within a reactor tank. The Perry et al. patent also describes recycling a portion of the spent catalyst solution directly back into the catalyst feed. In so doing, Perry et al. note that recycling the catalyst solution leads to a build up of water-soluble organic acid salts in the aqueous catalyst solution, and caution that "these salts inhibit the formation of the desired glycol monoester and the salt concentration must therefore be controlled." Perry et al. thus direct that the salt concentration in the catalyst solution be kept at less than 10% of the solution by weight and preferably less than 5 weight %. Following this work, U.S. Pat. No. 3,442,931 to Duke and Perry described a mixed, trimeric aldehyde condensation in which formaldehyde and aldehydes having only one alpha-hydrogen atom were contacted with a strongly basic catalyst at a temperature below 50.degree. C. Reaction times described in the examples are typically 3 hours or more, with other features of the reaction, including catalyst recycle and cautioned control of salt build up, being similar to those set forth in the above-described Perry et al. '821 patent
After this earlier work in the 1960's, researchers continued to study the aldehyde condensation reaction and set out to try to maximize productivity while maintaining acceptable monoester selectivities and yields. For example, U.S. Pat. No. 3,718,689 issued to McCain et al. in 1973 describes a process for producing high ratios of hydroxyalkyl alkanoate product with relation to alkanediol byproduct. The described process includes continuously subjecting, in a longitudinal flow zone, an intimate mixture containing aldehyde and smaller amounts of a relatively concentrated aqueous basic solution, e.g. aqueous alkali metal or alkaline earth metal hydroxides. Again, the alkali metal hydroxide, sodium hydroxide, is the preferred catalyst. McCain et al. state that residence periods up to about 2 hours give satisfactory results, and indicate that marked increases in productivity are obtained by maintaining a maximum residence period of up to approximately ten minutes. McCain et al. describe no attempt to recycle catalyst solutions, rather directing that the spent aqueous stream containing dissolved salts be removed and discarded.
Despite varying catalysts described in early and more recent literature, alkali metal hydroxides such as sodium hydroxide have remained the preferred catalysts in the field and industry to date. For example, more recently, U.S. Pat. No. 4,883,906 issued to Argyropoulos et al. in 1989 describes a process in which sodium hydroxide is the preferred catalyst. This process involves distilling the reaction product immediately after leaving the reactor and before it has cooled or been contacted with any added water. The patent states that this step significantly reduces byproducts and simplifies purification of the product. The patent also indicates the need to carefully control this initial distillation to prevent or minimize decomposition of the desired monoester product to diol and diester byproducts.
As to other catalysts which have been described for aldehyde condensations, U.S. Pat. No. 3,475,343 to Kusama et al. describes a process which forms a mixture of aliphatic carboxylic acid esters by subjecting a mixture of saturated aliphatic aldehydes or 4-alkoxy substitutes thereof to condensation in the presence of aluminum alcoholate catalyst. U.S. Pat. No. 4,273,934 to Merger et al. describes preparation of 3-hydroxy-2,2,4-trimethylpentyl isobutyrate using an alkaline earth metal hydroxide and carboxylic acids or the corresponding alkali metal or alkaline earth metal salts. U.S. Pat. No. 3,703,541 describes isobutyraldehyde condensation to the monoester in the presence of an alkali metal salt of a monohydric or polyhydric phenol; and, U.S. Pat. No. 4,225,726 describes condensations of aldehydes in the presence of tin metal or tin oxide catalyst.
In light of this extensive background, there remains a need for improved processes employing the preferred alkali metal hydroxide catalysts which can provide even greater monoester productivities while maintaining good yields and selectivities. Additionally, despite their frequent characterization as "byproducts", the diol and diester and other derivative products also enjoy demand in commerce. Accordingly, there is a need for processes which can effectively be used to control the relative amounts of the monoester, diol and diester products, to meet current market demands. The applicants' invention addresses these and other needs in its various embodiments, and provides surprising processes in which condensation reactions are highly and unexpectedly modified by the presence of alkalai metal isobutyrate salts.